Automatic in-circuit testers for testing components mounted on a printed circuit board ("PCB") are well known. Typically, an in-circuit tester comprises a board fixturing system that employs a plurality of probes arranged in a so-called "bed of nails" for establishing electrical contact between the probes and predetermined contact points on the PCB. Two well known automatic in-circuit testers that employ a bed of nails in the board fixturing system are the HP Model 3065 SMT and HP Model 3070 SMT automatic in-circuit testers manufactured by Hewlett Packard Company, Palo Alto, Calif., the assignee of the present application. The board fixturing system employed in the HP Model 3070 SMT automatic in-circuit tester is disclosed in U.S. Pat. No. 4,818,933, which is incorporated herein by reference.
Contact contamination between the individual probes of the bed of nails and the PCB under test has become more common. Contact contamination may cause a high resistance contact to be established between the probe and the contact point on the PCB, thus resulting in false indications of component failure. The reason for the increase in contact problems is due to changes in PCB soldering technology, as well as use of increased node densities in PCBs.
In the case of changes to PCB soldering technology, many PCB manufacturers have eliminated board wash systems that employ chlorofluorocarbons for environmental reasons. Elimination of the board wash system has resulted in a decrease in PCB cleanliness and, since reliable contact is directly related to board cleanliness, contact problems related to contamination are on the rise.
As mentioned, increased node count is another reason for false indications of failure. In a one hundred node fixture, one false failure in probing per thousand probe contacts occurs, on average, on ten percent of the PCBs tested. For a one thousand node fixture, this problem is increased ten-fold to one hundred percent. This means that, on average, no PCBs will pass the first time, due to false failures as a direct result of increased node count.
Several solutions to the contact contamination problem have been proposed. Increasing the spring force employed to urge the probe into contact with the PCB has proven to decrease probe contact problems. It has been found that probes that exert a force of greater than eight ounces provide the greatest benefit. However, probes that are capable of being exerted against the PCBs at these forces are not available in sizes smaller than those used for 0.100 inch probe spacing. Frequently, the geometry of surface mount PCBs is such that the vias and pads are on less than 0.100 inch centers. For high node count fixtures, the use of high spring force probes may not be possible on vacuum type fixtures due to limitations in the magnitude of atmospheric pressure available to force the PCB down against the probes. Additionally, use of higher probe forces reduces the life of the spring that exerts the force.
Double probing (i.e., placing two probes on each node) has also been found to be effective in decreasing false indications of component failure due to probe contact problems. Assuming that the two probes on each electrical node are statistically independent, the probability of a poor contact is the product of the error probability of each probe. For an individual error rate of one in one thousand, and with two probes on each node, the probability decreases to one in one million. For a one thousand node fixture, this translates to a false probing rate of one in one thousand PCBs tested. Double probing, however, is expensive to implement.
Another known solution is the use of a probe that is designed to twist or rotate as the probe is compressed. This style of probe, frequently known as a twist probe, is presently available only for probes that are mounted 0.100 inch centers. This style of probe is more expensive than conventional style compression probes.
U.S. Pat. No. 3,996,516 describes an apparatus that applies ultrasonic vibrations to a fixturing system, and hence to a test piece, ostensibly for the purpose of improving electrical conductivity at the points of contact between the test piece and measuring pins. However, a test conducted by applicants of the instant application has shown that application of ultrasonic vibrations to a PCB in a board fixturing system did not improve electrical conductivity between the probes and the PCB. Moreover, the apparatus described in the '516 patent suffers from the disadvantage that expensive equipment must be added (i.e., an ultrasonic generator) to the conventional fixturing system. Still further, the apparatus of the '516 patent is not suitable for testing components mounted on a PCB, since the objective of that apparatus is to test for continuity and contact resistances of 2-3 ohms are acceptable. However, in-circuit testing of a PCB's components requires contact resistances of less than 1 ohm.
U.S. Pat. No. 3,453,545 describes a modification to a fixturing system for integrated circuits wherein a wafer is vibrated relative to a plurality of probes, again ostensibly to improve the electrical conductivity therebetween. As in the case of the '516 patent, the apparatus described in the '545 patent suffers from the disadvantage that equipment must be added to a conventional fixturing system to cause the vibrations. Moreover, the apparatus described in the '545 patent is specifically directed to testing the wafers employed in integrated circuits and does not find practical use in in-circuit testing of components mounted on a PCB.
It is therefore desirable to provide a method and apparatus for use in connection with the board fixturing system of an automatic in-circuit tester that reduces problems associated with contact contamination but yet is simple, reliable and inexpensive to implement. The present invention achieves these goals.